1. Field of the Invention
This invention relates to a golf ball comprising a core and a cover having at least one thin spherical layer, and a method for manufacturing the same.
2. Prior Art
Solid golf balls constructed of a polybutadiene rubber core at the center and a spherical cover made of ionomer resin on the outside are in widespread use today. There has been a trend in recent years toward specific combinations of two or three layers in the cover, each made of a distinctive material. The aim is two-fold: to approach more closely a A level of performance which both incorporates such desirable features of thread-wound balls as their good spin receptivity and the easier control they allow over where the ball stops after it has been hit and also provides the longer carry of the solid balls, and to further increase the ball""s carry. Moreover, within the limit imposed by regulations on the outside diameter of the ball, the core which generates most of the rebound energy that powers the ball""s flight must have the largest volume possible. This set of circumstances has given rise to a need for thinner molded covers than in the past.
When solid golf balls first appeared, development began with two-piece balls made of a core and a one-layer cover. The cover was composed of a single layer of ionomer resin that was formed, separately from the core, into a pair of half-cups in a mold, then placed over the core and molded integrally with the core in another mold. For this reason, the thickness of the cover was generally set at about 2.0 mm (0.0787 in.). Later, covers came to be molded with injection molding machines by injecting resin about the periphery of a core positioned at the center of a spherical mold. Because the cover thickness was maintained at the preexisting value of about 2.0 mm (0.0787 in), there was no need to make the injection molding machine fill speed particularly rapid. Thus, given a melt flow rate (MFR) at 190xc2x0 C. of from 1 to 10 for the injection molding material, the standard practice has been to set the fill speed for molding one ball at from 10 to 20 cm3/s (0.6102 to 1.2205 in3/s).
However, as noted above, when the number of layers in the cover increases to two or more and it becomes necessary to reduce the thickness of individual layers, the injected layers of resin are thinner so that they cool more rapidly. At the above-indicated range of fill speed in the injection molding machine, curing begins to arise before the resin for a given layer has extended over the entire surface of the core. This raises the possibility that the thin layer will not have a uniform thickness over the peripheral surface of the ball.
Although attempts have been made to adjust the properties of the molding material and the mold fill speed by trial and error, not only is such adjustment difficult, excessive modification of these parameters sometimes leads to a decline in ball performance and an increase in the level of production defects.
It is therefore an object of the present invention to provide a golf ball, and a method for its manufacture, in which an injection molding material is injection molded at an optimum fill speed to form a thin spherical cover layer, thereby enabling good adaptability to future improvements in ball performance, minimizing production defects in golf balls having thin spherical layers, and ensuring that thin spherical layers of uniform thickness are formed from particular molding materials used.
The invention provides a golf ball comprising a core and a cover enclosing the core, the cover having at least one thin spherical layer with a thickness of not more than 1.5 mm (0.0591 in.). The thin layer has been injection molded from an injection molding material under conditions which satisfy formula (1):
Vxe2x89xa7(a/t2)xe2x88x92bxe2x80x83xe2x80x83(1)
wherein V is the injection molding material fill speed in cubic centimeters per second, t is the thickness of the thin spherical layer in millimeters, a=0.04M2xe2x88x923.75M+137.5, and b=xe2x88x921.49/Mxe2x88x920.57, and M is the melt flow rate at 190xc2x0 C. of the injection molding material for the thin spherical layer.
The invention provides also a method of manufacturing the foregoing golf ball, which method comprises injection molding the thin spherical layer from an injection molding material under conditions that satisfy above formula (1).
In the golf ball of the invention, a thin spherical layer is-injection molded about a core by means of an injection molding machine in which the filling velocity v (cm3/s) of the injection molding material has been set at a value not less than the lower limit value defined by formula (1) above. The thin spherical layer can thus be formed to a very thin and uniform thickness of not more than 1.5 mm (0.0591 in.), making it possible to obtain a golf ball having a large core diameter and a plurality of thin spherical cover layers. This ball construction provides a large coefficient of restitution that increases the carry of the ball and can also generate or maintain a spin rate close to that of a thread-wound ball.
To arrive at formula (1), injection molding was repeatedly carried out using materials having various melt flow rates (MFR) while successively reducing the injection molding machine fill speed for each of various gap sizes between the inner face of the injection mold and the core, representing different thin spherical layer thicknesses, set by varying the core diameter, and the filled state of the thin spherical layer in each case was investigated. Based upon the results obtained, a limit fill speed was determined at which there do not arise areas in the mold unfilled with the thin spherical layer material or marked irregularities in the thickness of the thin spherical layer. Accordingly, such unfilled areas and thickness irregularities do not occur when injection molding is carried out using a fill speed equal to or greater than the value calculated in formula (1).
The melt flow rate (MFR) 1 ng/10 mm represents the ease with which a molding material that has been heated and melted flows within the runners in the mold when forced out by the injection molding machine. This value is measured with a commercial melt indexer. A higher value indicates that the material flows more easily.
Based on correlation diagrams obtained from our experiments, we have found that, to produce thin spherical cover layers in the manufacture of golf balls, the optimal practical range in the melt flow rate at a material temperature of 190xc2x0 C. is 0.5 to 50. Our results confirm that at a melt flow rate of less than 0.5, the material flows with greater difficulty, making it necessary to set the injection molding machine to an extremely high fill speed. On the other hand, a melt flow rate of more than 50 may result in the formation of numerous burrs. Accordingly, it is preferable for the melt flow rate of the injection molding material used to form the thin spherical layer in the golf ball of the present invention to be selected within a range of 0.5 to 50.
In the manufacture of high-performance balls, the fill speed V (cm3/s) of the injection molding machine which injects the material that forms the thin spherical cover layer is defined by formula (1):
xe2x80x83Vxe2x89xa7(a/t2)xe2x88x92bxe2x80x83xe2x80x83(1)
wherein t is the thickness of the thin spherical layer in millimeters, a and b are variables dependent on the thickness t (mm) of the thin spherical layer molded and the melt flow rate of the material. Hence, areas that are incompletely filled with the thin spherical layer material and irregularities in the thickness of this layer do not arise on the ball periphery due to an insufficient fill speed. This also eliminates excessive adjustments in material properties in an attempt to increase the fluidity of the molten material, thereby making it possible to reduce production defects and prevent a decline in ball performance.
While the lower limit value in the fill speed is fixed by above formula (1), the fill speed may be set at any value above this lower limit that can be empirically determined in accordance with the injection capability and mold capability (including heating capability, thermal conductivity, type of runner system, and number of balls produced per mold) of the injection molding machine used.